Wound healing may be understood to occur in four phases: (1) Inflammation and hemostasis, (2) proliferation, (3) remodeling, and (4) epithelialization. Inflammation may be understood to initiate with an injury itself. Due to the disrupted blood vessel(s), bleeding may result in the release of (a) platelets to form clot(s) and inhibit bleeding, (b) white blood cells to clear wound debris, and (c) proteases/collagenases to degrade wounded portions of the extracellular matrix. In the proliferation phase, endothelial cells may be understood to migrate from nearby intact venules and start proliferation to form new vessels that supply the rebuilding cells with oxygen and nutrients. In this period, stem cells and fibroblasts may be recruited to the wound site to produce a new extracellular matrix framework, primarily collagen. In the next remodeling phase, the strength of collagen framework may be enhanced. In the final epithelialization phase, a new skin may be formed.
Important aspects of the wound healing process include re-vascularization and collagen deposition. Re-vascularization, or neoangiogenesis, to re-establish tissue perfusion and supply nutrition and oxygen to the wound bed may be understood as the foundation of the wound healing process. Anti-bacterial lymphocytes do not work well to clear the wound sites in hypoxic environment. Furthermore, oxygen supply may be understood to be critical for the increase of cell metabolism to synthesize and deposit collagen to fill the wound defects. Moreover, wound closure through collagen deposition may aid the healing process and protect against invading pathogens, as a wound may not have an effective skin barrier to act as a first line of defense.
Wounds which may be considered particularly difficult to heal include chronic wounds and combat wounds. Chronic wounds represent a significant and growing challenge to our healthcare system. Chronic wounds, such as venous leg ulcers, pressure ulcers, and foot ulcers in diabetic patients, often fail to achieve adequate healing. Failure to achieve adequate healing is often related to tissue ischemia due to poor local blood supply. These chronic wounds may be seen in ambulatory wound care centers with frequent recurrences yet less satisfactory outcomes, and therefore need long term care, resulting in growing utilization of health care resources. If tissue ischemia and hypoxic condition could be altered regionally, an increased healing rate could be achieved. For example, nitric oxide presented at wound sites may send a vasodilatation signal which results in an increase of local blood flow and the acceleration of collagen synthesis and wound closure. Similarly, studies indicated that the application of aloe vera at the wound sites also promotes blood flow and results in an increased oxygen supply.
Combat wounds, such as those due to high energy blasts, may be characterized by severe composite tissue damage, large zones of injury, and extensive vascular disruption. Due to the difficult closure of these large wounds, one of the most serious complications of these wounds may be infection. Consequently, the longer the wound is open, the more difficult it often may be for the wounds to heal. When a damaged area becomes infected, the infection may be understood to stop the healing process and can even cause poisoning and death. Optimal care and reconstruction of these massive soft tissue and bone defects of the extremities and craniofacial skeleton has yet to be adequately defined.
Tissue transplantation may be one treatment used to augment deficient tissue in the region of injury and therefore cover the large area of wounds. One tissue transplantation procedure in craniofacial and extremity wounds may involve microsurgical free flap reconstruction. This procedure may be understood to completely detach skin, fat, and blood vessels (arteries and veins), called a flap, from one part of the body and move them to the wound sites. The flap may then be reattached, with the arteries and veins of the flap reconnected to the arteries and veins near the wound sites. However, free flap reconstruction is limited by the availability of the tissue flaps from either autograft or allograft due to the donor-site morbidity. Thus, while free flap reconstruction may be used in the treatment of chronic and combat wounds, such reconstruction is somewhat limited.
Tissue engineering and regenerative medicine may also be used for treating traumatic tissue damage and loss. Engineered tissue scaffolds may include natural and synthetic materials such as collagen, hydroxyapatite (HA) and PLGA. Collagen-based acellular scaffolds may be particularly used for wound treatment. Three-dimensional scaffolds may not only provide coverage over soft tissue deficit to reduce the risk of contamination and subsequent infection, but also may regulate MMP (matrix metalloprotease) levels to promote normal progression through the stages of wound healing. In comparison with acellular scaffolds, scaffolds pre-loaded with tissue specific cells and stem cells, which may be referred to as cell constructs, have a better chance to mimic functionality and complexity of native tissue, and thus achieve a better and faster wound treatment.
Engineered cell constructs may be used to repair cartilage, skin, urethra, and blood vessels. However, the success of these constructs in vivo is understood to be mainly due to their low thickness, where oxygen and nutrients can diffuse into the constructs and sustain cellular viability. However, as the constructs become larger and thicker, cells located more than 200 to 300 μm away from nearest capillaries may be understood to suffer from hypoxia and apoptosis following the implantation. It usually takes days or even weeks before the host's vasculature can grow into the constructs to feed the cells inside the constructs. At that time most of the preloaded cells on the constructs will die. Therefore, the implantation of cell constructs for wound repair remains a challenge.
Attempts have been made to get blood supply close to cell constructs after implantation. One attempt was to form an open groove in an outer surface of a tricalcium phosphate scaffold and thereafter a nearby artery and vein of the host was inserted into the groove of the scaffold after it was implanted. Another attempt was to place a cell construct inside a rigid, (inflexible) chamber. A nearby artery and vein was then sutured to the chamber to supply blood to the scaffold in what may be referred to as an arteriovenous (AV) loop. The concept of an AV loop derives from the free flap reconstruction technique used in clinics. Although both examples may increase of local perfusion to the scaffolds, they have several problems. First, both are still diffusion models, as there are no integrated vascular paths that can directly connect to blood supply to deliver blood to the cells deep inside the scaffold upon the implantation. Second, in the case of the rigid chamber, the chamber actually prevents the cell construct from fully interacting with surround tissues and finally integrating into the existing tissue to reach the goal of wound healing. Third, the rigid chamber is not easy to adapt to different shapes of constructs.
Thus, while engineered “tissue flaps” may be used for wound care, the survival of the cell construct after implanted in vivo remains a challenge, particularly due at least in part to perfusion and associated hypoxia.